The field of ink-jet printing is replete with references describing solutions to problems associated with placing ink drops on a print medium. In particular, color ink-jet printing requires careful placement of ink drops to meet print resolution and color fidelity requirements without producing undesired printing artifacts such as banding, streaking, bleeding, puddling, and chroma shifting.
Ink drop placement-related problems vary in severity with a large number of printer-related variables including desired printing speed, print head array configurations, bidirectional versus unidirectional printing, transfer versus direct printing, aqueous versus phase change ink, the printing resolution required, print postprocessing employed, if any, and the type of print medium employed. Solutions to the above-mentioned problems are often associated with particular sets of printing techniques, such as: print interlacing to avoid banding and streaking artifacts; dithering patterns to improve printed image color gamut and gray scale; ink color laydown patterns to reduce bleeding and puddling; ink color laydown sequences to compensate for printing direction and color shifts; ink drops sizes and cross-sectional profiles adapted for particular print medium types and print processing techniques; specialized print media compatible with particular inks; and print postprocessing to improve printed image durability, appearance, and projectability. Many prior print interlacing methods and print head nozzle array patterns are known because of the correspondingly wide variety of nozzle array configurations, ink types, print media supports, print head and media movement mechanisms employed by ink-jet printers. Interlacing methods may be further classified into band and line interlacing methods.
Color band interlacing refers to the partial overlapping of a first printed band of a color with a subsequent printed band of the same color. This also requires line interlacing and results in the spacing apart of any printing defects due, for example, to a defective ink jet in an array of ink jets.
Line interlacing entails printing adjacent lines of dots of a particular color during sequential scans of the print head. For example, lines 1, 3, 5, etc., are printed during a first scan, and lines 2, 4, 6, etc., are printed during the next scan. In a bidirectional printer, it is desirable to print during both scanning directions. Line interlacing causes printing errors and related image defects that are dependent on the scanning direction to be generated at a high spatial frequency that is the inverse of the spacing between lines. Such defects are not easily perceived by a human eye.
It is also known that the ink color laydown sequence is important and depends on the print head scanning direction, ink composition, and time between depositing successive drops. To reduce hue-related printing artifacts, ink laydown sequences should always be the same regardless of scan direction. If this is not possible, an alternative is to alternate the ink laydown sequences on adjacent lines so that the resulting hue variations will have a high spatial frequency that is not easily perceived by the human eye.
In some instances, prior workers have sought solutions to a common printing problem but have reached contradictory solutions. For example, when printing with phase change ink, some workers teach that print quality is optimized when adjacent ink drops are allowed to melt together, or coalesce, and other workers teach that adjacent ink drops should not coalesce.
Teaching that adjacent ink drops should coalesce is found in U.S. Pat. No. 5,075,689 issued Dec. 24, 1991 for BIDIRECTIONAL HOT MELT INK JET PRINTING, which describes a phase-change ink-jet printer in which printed color hue is dependent on the order in which inks are deposited one on top of the other. If a first colored ink drop is deposited and a second colored ink drop is deposited on top of the first drop, a particular color is created. But if the ink color laydown sequence is reversed, a slightly different color is created. The patent proposes depositing both drops in such a short time period that they remain in a liquid state that allows their colors to mix together prior to setting. However, this solution is not satisfactory for all phase-change inks, especially those having high chromaticity. Moreover, because pairs of liquid drops that coalesce form a larger resultant drop than that in which the second drop is deposited on top of a set drop, color hue shift effects are still noticeable.
The contradictory teaching is found in U.S. Pat. No. 5,070,345 issued Dec. 3, 1991 for INTERLACED INK JET PRINTING which characterizes many of the banding and seaming problems associated with phase-change ink-jet printing and describes guidelines for minimizing those problems by preventing adjacent ink drops from coalescing. The guidelines state that banding can be minimized if adjacent dot rows are not printed during the same pass, and each dot row should be deposited between either unprinted adjacent dot rows or deposited between adjacent printed dot rows. Thereby, printing artifacts caused by ink blending and thermal unbalance problems are minimized.
Other workers have proposed ink drop laydown patterns as solutions to particular print quality problems. For example, U.S. Pat. No. 4,967,203 issued Oct. 30, 1990 for INTERLACE PRINTING PROCESS and U.S. Pat. No. 4,999,646 issued Mar. 12, 1991 for METHOD FOR ENHANCING THE UNIFORMITY AND CONSISTENCE OF DOT FORMATION PRODUCED BY COLOR INK JET PRINTING describe color liquid ink-jet printing methods in which predetermined ink drop patterns are employed to reduce liquid ink bleeding, coalescence, hue shift, and banding problems on transparency, plain paper, and special media. The patents describe staggering and alternating the ink drop laydown patterns such that overlapping liquid ink dots are allowed to dry because they are printed on alternate passes of a print head. Also described are "super pixels" of four pixels each whereby printed color hue is improved by employing predetermined ink color laydown and drying sequences for each super pixel.
Another ink drop laydown pattern is described in European Pat. Application No. 0 476 860 A2 published Mar. 25, 1992 for INK DROP PLACEMENT FOR IMPROVED IMAGING in which liquid ink-jet images are improved by printing groups of adjacent drops as clusters of overlapping ink drops that uniformly coalesce toward the center of each cluster. The ink drop cluster pattern substantially eliminates random ink drop coalescence that causes a mottled image. Such mottling is particularly observable when printing on transparency films.
Still other workers have proposed image postprocessing as a solution to transparency print quality problems. In particular, phase change ink-jet printing on transparency film causes individual ink drops to solidify into a lens-like shape that disperses transmitted light rays, resulting in a very dim projected image. This problem is generally solved by postprocessing the phase change ink image with some combination of temperature and pressure to flatten the ink drops. For example, U.S. Pat. No. 4,889,761 issued Dec. 26, 1989 for SUBSTRATES HAVING A LIGHT-TRANSMISSIVE PHASE CHANGE INK PRINTED THEREON AND METHODS FOR PRODUCING SAME, which is assigned to the assignee of this application, describes passing a print medium through a nip formed between two rollers at a nip pressure of about 3,500 pound/inch.sup.2 ("psi") to flatten the ink drops and fuse them into the pores and fibers of the print medium. Controlled pressure in the nip flattens the ink drops into a pancake shape to provide a more light-transmissive shape and to achieve a degree of drop spreading appropriate for the printer resolution. The roller surfaces may be textured to emboss a desirable reflective pattern into the fused image. Unfortunately, such rollers are expensive, bulky, may provide nonuniform fusing pressure, and can cause print medium deformations.
Printing on transparency film also suffers from ink-related problems. The dye concentration in many inks is limited by environmental and health concern-induced regulations. For transparent inks, such as phase change inks, a dye concentration suitable for adequately color-saturated plain paper images will produce a half saturated, washed-out appearing image on transparency film. This is because with plain paper, light passes through the image, reflects off the paper and back through the image, making two passes through the ink whereas with transparency film light makes a single pass through the ink. A prior solution is to print the transparency image twice to increase the color saturation. Unfortunately, such multilayer phase change ink images can be unsuitable for use because the secondary colors are distorted, banded, bumpy, and have ink strands and multiple pixel displacements that can occur during transfer printing or image postprocessing.
Multilayer printing with phase change inks is also a problem on nontransparent print media because of secondary color bands that result from poor registration of overlayed ink drops. For example, a blue primary color is produced by exactly registering a drop of magenta ink and a drop of cyan ink. To the degree the drops are misregistered, the nonoverlapping portions of the magenta and cyan ink drops will be visible.
Clearly, the above-described mix of problems has a large number of prior solutions, some of which are contradictory and depend on the particular combination of printing technology employed.
What is needed, therefore, is a color ink-jet printing apparatus and method that provide high color-saturation image printing on transparency film and other print media without visible color shifts, image artifacts, or significant light dispersal, and without requiring image postprocessing.